cities of Koper, Slovenia, and Trieste, Italy, via G11 in Slovenia. The first section of the D21 route followed an overall eastern orientation. About away from the borderline, it reaches the Kaštel border crossing, and after , there is an at-grade intersection where the traffic defaults to D510, which serves as a connector to the A9 motorway. D21 itself turns southwest towards the village of Kaštel, approximately away. Just to the north of Kaštel, D21 turned south before proceeding to the town of Buje located about down the road from Kaštel. In Buje, D21 forms at-grade intersections with D200 and D300. D200 provides a route to the Plovanija border crossing with Slovenia and D300 to the town of Umag and Umag interchange of A9 motorway. There are also at-grade intersections with the Ž5007 and Ž5008 county roads in the town. South of Buje, the route turns southeast across hilly terrain until it reaches Ponte Portone in the Mirna River valley approximately away from Buje. There D21 formed a roundabout junction at the north bank of the river. The roundabout connected D21 to D44, which links the town of Buzet to the east, and to D301 which links the town of Novigrad and another A9 interchange to the west. After the route crossed the Mirna River, it continued about southeast to Vižinada, where D21 forms an intersection with Ž5041 county road to Kaštelir and Poreč. After Vižinada, the route continued another south, where it reached the Ž5042 intersection connecting Višnjan. Approximately south, at Baderna, D21 connects with D302 extending to Poreč and the A9 motorway, located west of the road junction, as well as D48 leading to Pazin east of D21. The intersection is built as a partial cloverleaf interchange. Approximately south of Baderna, the route reached the village of Sveti Lovreč and continued south to the Medaki interchange of the A9 – another partial cloverleaf interchange – about south of Sveti Lovreč. D21 turned southwest at the Lim embayment of the Adriatic Sea after another . At that point, D21 formed an at-grade intersection with Ž5002 road to Vrsar situated further west. Following the Ž5002 junction, D21 turns southeast to follow the contours of the Lim embayment valley. After approximately , the route reached a grade separated intersection, a two-ramp partial cloverleaf, with D303. D303, in turn, links to the Kanfanar interchange of the A9 motorway and the coastal city